The electroless deposition of metals onto substrates, e.g. polymeric thermoplastic or elastomeric substrates, is often achieved by first sensitizing the substrate to electroless deposition by application of a catalytic material, e.g. a palladium colloid or complex. A common deficiency of laminates comprising electrolessly deposited metal on plastic substrates is poor adhesion of the metal layer to the polymeric substrate, resulting from the poor adhesion of the sensitizing catalyst. Consequently, attempts to improve adhesion of electrolessly deposited metal layers is often focused on improving the adhesion of the catalytic material to the polymeric substrate, e.g. by using acid, base or solvent to etch or swell the surface. Such techniques often do not provide satisfactory adhesion, for instance, because substrate surfaces tend to degrade when treated with acid, base or solvent or because the substrate surface is resistant to such treatment.
Another method, e.g. providing films having catalytic metal particles incorporated into a polymeric binder, generally provides metal coatings of poor quality in terms of appearance and adhesion.
Still another method of providing substrates with adherent catalytic surfaces capable of providing substantially adhering metal layers of high quality involves the catalytic activation of inert polymer films prepared from solutions of catalytic metal, e.g. palladium, and polymer. Morgan et al. in U.S. Pat. No. 4,910,072 disclose such films prepared from organic solvents; an environmental disadvantage of such films is the high volume of organic solvent that is generated in producing such films. More environmentally advantageous are the catalytic films disclosed by Vaughn in U.S. Pat. No. 5,082,734; for instance, catalytic films are prepared from water soluble polymers, e.g. cellulose derivative polymers and polyvinyl alcohol, and aqueous emulsions of polymers. A disadvantage of such films comprising water soluble polymers is poor adhesion to selected substrates, e.g. polyurethane.
The polymer films used in this invention utilize sulfonated perfluorocarbon polymers; such polymers are commonly fabricated into Nafion.RTM. cation permeable membranes available from E.I. du Pont de Nemours & Company for use in electrochemical devices such as fuel cells, batteries, and other applications requiring cation permeability. For such membrane applications it is common to provide metal coatings, e.g. as electrodes, on the membrane surface. Various methods for applying metal coatings to sulfonated perfluorocarbon polymer membranes are disclosed in the Final Report by Panclor, S.p.A., Milan, Italy to the Commission of the European Communities, Directorate-General for Research, Science and Education, entitled "Study of the Optimal Structure of a Solid Polymer Electrolyte For Water Electrolysis" (Contract No 703-79-1-EHI). Panclor's methods for the deposition of nickel onto sulfonated perfluorocarbon polymer membranes include: (a) treating with sodium sulfide and then nickel sulfate to form a nickel sulfide conductive layer onto which nickel is electroplated; (b) treating with tin chloride and then silver nitrate and formaldehyde to form a silver layer onto which nickel is electroplated; and (c) treating with tin chloride and then palladium chloride and sodium hypophosphite to form a palladium layer onto which nickel is electrolessly deposited. The latter method is reported to have provided better quality nickel electrodes in terms of electrochemical performance and durability as determined by a bending test and thermocycle test.
Fujita et al. in Japanese Kokai Patent Publication Sho 61[1986]-87887 discloses the preparation of electrodes on Nafion.RTM. sulfonated perfluorocarbon polymer membranes. An electrode is formed on one surface of the membrane by electroless plating of rhodium. An electrode is formed on the other surface of the membrane by hot pressing a mixture of black platinum powder, sulfonated styrene-divinylbenzene resin powder, an alcohol solution of Nafion.RTM. sulfonated perfluorocarbon polymer and a suspension of polytetrafluoroethylene.
Kiyoya et al. in Japanese Kokai Patent Publication Sho 58[1983]-83030 discloses providing electrodes on perfluorocarbon polymer ion exchange membranes by impregnating a sulfonated surface with a platinum salt, reducing with sodium borohydride and then electrolessly depositing platinum.
Because it is difficult to adhere such membranes to substrates and because the metal coatings are generally porous, e.g. to accommodate fluid flow through the membrane, laminates of metal coated sulfonated perfluorocarbon polymer membranes are not particularly useful for applying metal coatings onto other substrates.
Sulfonated perfluorocarbon polymers have also been used to provide thin coatings onto articles. See Grot's disclosure in U.S. Pat. No. 4,433,082 of sulfonated perfluorocarbon polymer solutions which are useful to repair damaged membranes and provide ion exchange coatings. According to Moore et al., Macromolecules, 1988, 21, 1334-1339, films of solution-cast sulfonated perfluorocarbon polymers exhibit different physical and chemical properties than commercial membranes; for instance, the solution-cast films are not as pliant and mechanically strong and are also brittle and highly soluble. Moore et al. attribute these poor properties to a lack of crystallinity in the polymer film. Crystallinity can be imparted into the polymer film by changing the solvent from which films are cast or by heat treatment, e.g. above the matrix polymer glass transition temperature (about 140.degree. C. for sodium forms of the ionic polymer). See also Kinlen et al. who disclose in U.S. Pat. No. 4,818,365 procedures for coating electrodes with cation permeable coatings of perfluorosulfonic acid polymer which include dipping electrodes in solutions of the polymer and low boiling point solvents, such as water and lower alkyl alcohols, evaporating the solvent at less than 120.degree. C., and then annealing the polymer coating preferably at a temperature of about 180.degree.-230.degree. C.
An object of this invention is to provide catalytic films that allow the electroless deposition of strongly adhering metal layers on substrates without acid, base or solvent treatment of the substrate surface.
Another object of this invention is to provide catalytic films using environmentally acceptable solvents, e.g. water, alcohols and mixtures thereof.